JP3062275B2 - Steel for high strength shaft parts - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a carbon steel for machine structural use which is excellent in cold workability and induction hardenability as hot rolled, and more particularly to a steel suitable for use as a drive shaft for automobiles.

[0002]

2. Description of the Related Art Conventionally, 0.4 wt% C alloy steel has been used for shaft parts such as drive shafts for automobiles.
A processing method in which a hot-rolled bar is annealed to reduce its strength, subjected to cold working such as rolling and cutting, and then subjected to induction hardening and tempering to secure important torsional strength as a shaft part. Was common.

However, there are problems in cost and productivity, such as the necessity of annealing before cold working, and the fact that annealing may reduce the hardenability, making it impossible to effectively use alloy elements. Was leaving. To solve such a problem,
After rolling and cutting without annealing and reducing the amount of alloying elements, induction hardening and tempering are carried out, and the method is changing to a method of securing necessary characteristics as a shaft part.

On the other hand, in recent years, there has been a strong demand for automotive parts to be lighter in weight due to environmental problems, particularly, a demand for reduction of CO ₂ gas. In the case of shaft parts, improvement of torsional strength, which is the most important product characteristic, is required from the viewpoint of weight reduction. In general, in order to increase the torsional strength, a method of increasing the quench hardening depth by induction hardening has been adopted.

In order to increase the quench hardening depth, it is conceivable to change the induction hardening conditions, in particular, to reduce the frequency of the high frequency induction heating apparatus or to increase the amount of alloying elements in the steel material. In order to reduce the frequency of the device, it is necessary to purchase a new and expensive high-frequency oscillator, which is not economical. On the other hand, when the amount of alloying elements is increased, the strength of the hot-rolled steel bar, which is the material, increases, and if it is used as it is, it causes problems in cutting and rolling, so annealing is necessary. , Economy and productivity.

Further, it is not efficient to achieve high strength only by increasing the quenching depth (x / r, x: thickness of the quenched layer, r: radius of the shaft material), and x / r becomes zero. Even if the quenching depth is increased beyond 0.5, the strength is not significantly increased, and the rate of occurrence of quenching cracks increases as shown in FIG. 1, which is a problem in stable production and quality assurance.

[0007]

SUMMARY OF THE INVENTION The present invention relates to a shaft member having a high torsional strength, a long tool life during cold working and not requiring annealing before working, ie, induction hardening and cold workability. An object of the present invention is to provide an excellent steel material for high-strength shaft parts, and to solve the problems in the prior art in terms of cost and productivity.

[0008]

According to a first aspect of the present invention, C: 0.35 to 0.60 wt% Si: 0.05 wt% or less Mn: more than 0.65 to 1.70 wt% P: 0.1 to 0.6 wt% 020 wt% or less S: 0.005 to 0.035 wt % Cr: 0.50 wt% or less Ti: 0.01 to 0.05 wt% B: 0.0003 to 0.0050 wt% N: 0.005 wt% or less O: 0 0.002 wt% or less Fe and unavoidable impurities: The balance, and the content of C, Si, Mn , and Cr simultaneously satisfies the following formula: IH = 161 0.4 × C ² -145.6 × C ³ -27.6 ×
C + 0.01 × Si + 4.10 × Mn + 2.75 × Cr ≧ 10.0 IT = 78.7 × C + 17.7 × Si + 24.9 × Mn
+ 22.5 × Cr + 4.19 ≦ 70.2 C: C content (wt%) Si: Si content (wt%) Mn: Mn content (wt%) Cr: Cr content (wt%) . In the second invention of the present invention, C: 0.35 to 0.60 wt% Si: 0.05 wt% or less Mn: more than 0.65 to 1.70 wt% P: 0.020 wt% or less S: 0.005 to less 0.035 wt% Cr: 0.50 wt% or less Mo: 0.05 to 0.50 wt% Ti: 0.01 to 0.05 wt% B: 0.0003 to 0.0050 wt% N: 0.005 wt% or less O: 0.002 wt% or less Fe and unavoidable impurities: contains the remainder, and C, Si, Mn, Cr, high strength shaft part for steel material content of Mo and satisfies the following expression at the same time To provide.

Note IH = 161.4 × C ² -145.6 × C ³ −27.6 × C + 0.01 × Si + 4.10 × Mn + 2.75 × Cr + 3.75 × Mo ≧ 10.0 IT = 78. 7 × C + 17.7 × Si + 24.9 × Mn + 22.5 × Cr + 19.14 × Mo + 4.19 ≦ 70.2 C: C content (wt%) Si: Si content (wt%) Mn: Mn content ( Cr: Cr content (wt%) Mo: Mo content (wt%)

[0010]

The present inventors have made intensive studies on the chemical composition of a steel material that has a small increase in torsional strength and a short tool life during cold working while suppressing the increase in quenching depth below the limit, and has achieved the present invention. It was done. Hereinafter, the present invention will be described in detail. First, the reasons for limiting the composition will be described. C: In order to improve the torsional strength, it is an essential component and is actively utilized. However, if the torsional strength is less than 0.35 wt%, the quench hardening depth is drastically increased. In this case, since the occurrence of sintering cracks becomes remarkable, the content is set to 0.35 wt% or more. Also, the upper limit is 0.60
If the content is more than 0.60 wt%, the toughness of the central part is deteriorated. Si: It has no effect on the hardenability of the steel material, and on the other hand, has a function of reducing the tool life at the time of rolling. Therefore, it is desirable to reduce it as much as possible, but up to 0.05 wt% is allowable. Mn: an element that improves the hardenability of steel, and at the same time, S in steel
Is an element that fixes hot brittleness. 0.65w
If it is less than t%, the effect of improving hardenability is small, and 1.70 w
If the content exceeds t%, the tool life at the time of rolling is reduced, so the content is limited to the range of more than 0.65 to 1.70 wt%. P: P is an element that promotes quenching cracking during quenching, so it is desirable to reduce it as much as possible, but it is allowable up to 0.020 wt%. S: An element that reduces cold workability, but is also an element useful for machinability and satisfies both properties.
The range is limited to 035 wt%. Cr: an element useful for hardenability and is preferable for this application, but is an expensive element, so it is used when Mn alone is not sufficient for hardenability. Therefore, the upper limit is set to 0.50 wt%. Ti: Since it is an element that sufficiently exerts the effect of improving the hardenability of B by being combined with N to form TiN, it is positively added, but if it is less than 0.01 wt%, the effect is small. If the content exceeds 0.05 wt%, a large amount of TiN is formed and is harmful to fatigue characteristics.
Limited to the range of 5 wt%. Mo: It is an element useful for hardenability and is preferable for this application but is an expensive element. Therefore, it is used when the hardenability is insufficient even if Mn is added. Is small and the effect is saturated even if it is added in excess of 0.50 wt%, so the content is set to 0.05 to 0.50 wt%. B: Since it is an element that improves the hardenability by adding a trace amount, it is added positively, but its effect is small when it is less than 0.0003 wt%, and when it exceeds 0.0050 wt%, the effect of B addition is conversely. Decreases from 0.0003 to 0.
It is limited to the range of 0050 wt%. N: It is desirable to reduce as much as possible, since the effect of improving the hardenability of B is reduced by forming BN by combining with B.
Up to 0.0050 wt% is allowed. O: Oxide-based non-metallic inclusions, which are harmful to fatigue characteristics, are desirable to be reduced as much as possible.
Up to t% is allowed.

When Cr is contained, IH = 161.4 × C ² -145.6 × C ³ -27.6 ×
C + 0.01 × Si + 4.10 × Mn + 2.75 × Cr ≧ 10.0 IT = 78.7 × C + 17.7 × Si + 24.9 × Mn
+ 22.5 × Cr +4.19 ≦ 70.2 consisting of IT and IH simultaneously, if the inclusion of M o ^{is, IH = 161.4 × C 2 -145.6} × C 3 -27.6 ×
C + 0.01 × Si + 4.10 × Mn + 2.75 × Cr + 3.75 × Mo ≧ 10.0 IT = 78.7 × C + 17.7 × Si + 24.9 × Mn
+ 22.5 × Cr + 19.14 × Mo + 4.19 ≦ 70.2 IH and IT must be satisfied at the same time. [C:
C content (wt%), Si: Si content (wt%), M
n: Mn content (wt%), Cr: Cr content (wt
%), Mo: Mo content (wt%)] As shown in FIG. 2, from the relationship between IH and torsional strength, when IH is less than 10.0, a target torsional strength of 140 kg / mm ² or more is obtained. Therefore, at least IH needs to be 10.0 or more.

Next, for various steel materials subjected to the induction hardening treatment, the relationship between the tool life at the time of rolling and IT is shown in FIG.
Shown in The tool life gradually decreases with the increase of IT.
When T exceeds 70.2, the tool life decreases rapidly, and IT
Is about 10-20% of the tool life when 55 is
Since the tool life during rolling is extremely shortened, the IT is set to 70.
2 or less.

Therefore, if the chemical composition cannot satisfy both IH and IT at the same time, both the characteristics of induction hardening and tool life during rolling cannot be satisfied, and the effects of the present invention can be fully exhibited. I can't get it.

[0014]

The present invention will be described below with reference to examples. 30
t A steel having a chemical composition shown in Tables 1 to 6 was melted by an electric furnace, passed through continuous casting and billet rolling, and then rolled to a diameter of 32.
mm straight bar. After performing serration, induction hardening, and tempering by rolling using this steel bar as a material, the results of investigation of the quenching depth, torsional strength, and tool life during rolling of the steel material are shown in Tables 2, 4, and 4. Also described in No. 6. The tool life ratio is shown as 1 for the conventional steel (No. 28 ).

[0015]

[Table 1]

[0016]

[Table 2]

[0017]

[Table 3]

[0018]

[Table 4]

[0019]

[Table 5]

[0020]

[Table 6]

No. 1 to No. 17 is the steel of the present invention,
When IH and IT are satisfied, and as a result, the torsional strength becomes 140 kgf / mm ² or more, the tool life is relatively good. No. 18 to 22 are cases where IT is not satisfied, and the torsional strength is 140
Although it is not less than kgf / mm ² , the tool life ratio is extremely reduced.

No. 23 to 26 are cases where any of Ti, B and N are out of the range of the present invention, and although IH and IT are satisfied, B does not act effectively at the time of quenching, and as a result, the quenching depth Has low torsional strength. No. 2
Reference numeral 7 is a steel material for drive shafts that is currently used, but has a high IT and a very short tool life. As a result, it is customary to use it after annealing before rolling. No.
27 tool life and torsional strength when subjected to annealing N
o. As shown in FIG. 28 , the tool life is high, but the torsional strength is low.

No. No. 29 is No. 27 is an improved material without annealing, but has low torsional strength.

[0024]

According to the present invention, a high-strength steel material for shaft parts can be produced without impairing productivity and cost, and the industrial use value is great. The steel of the present invention can be used not only for shaft parts but also for various mechanical parts.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the quenching depth and the rate of occurrence of quenching cracks.

FIG. 2 is a diagram showing a relationship between IH and torsional strength.

FIG. 3 is a diagram showing a relationship between IT and a tool life ratio.

Continuation of the front page (72) Inventor Shozaburo Nakano 1 Kawasaki-cho, Chiba-shi Kawasaki Steel Corporation Research and Development Headquarters (56) References Japanese Patent Publication No. Sho 63-62571 (JP, B2) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (2)

(57) [Claims] 1. C: 0.35 to 0.60 wt% Si: 0.05 wt% or less Mn: more than 0.65 to 1.70 wt% P: 0.020 wt% or less S: 0.005 to 0.035 wt% Cr: 0.50 wt% or less Ti: 0.01 to 0.05 wt% B: 0.0003 to 0.0050 wt% N: 0.005 wt% or less O: 0.002 wt% or less Fe and unavoidable impurities: the balance A steel material for a high-strength shaft part , wherein the contents of C, Si, Mn , and Cr simultaneously satisfy the following formulas. Note IH = 161.4 × C ² -145.6 × C ³ -27.6 ×
C + 0.01 × Si + 4.10 × Mn + 2.75 × Cr ≧ 10.0 IT = 78.7 × C + 17.7 × Si + 24.9 × Mn
+ 22.5 × Cr + 4.19 ≦ 70.2 C: C content (wt%) Si: Si content (wt%) Mn: Mn content (wt%) Cr: Cr content (wt%) 2. C: 0.35 to 0.60 wt% Si: 0.05 wt% or less Mn: more than 0.65 to 1.70 wt% P: 0.020 wt% or less S: 0.005 to 0.035 wt% Cr: 0.50 wt% or less Mo: 0.05 to 0.50 wt% Ti: 0.01 to 0.05 wt% B: 0.0003 to 0.0050 wt% N: 0.005 wt% or less O: 0.002 wt% A steel material for high-strength shaft parts, characterized by containing Fe and inevitable impurities: the balance, and wherein the contents of C, Si, Mn, Cr , and Mo simultaneously satisfy the following expression. Note IH = 161.4 × C ² -145.6 × C ³ -27.6 ×
C + 0.01 × Si + 4.10 × Mn + 2.75 × Cr + 3.75 × Mo ≧ 10.0 IT = 78.7 × C + 17.7 × Si + 24.9 × Mn
+ 22.5 × Cr + 19.14 × Mo + 4.19 ≦ 70.2 C: C content (wt%) Si: Si content (wt%) Mn: Mn content (wt%) Cr: Cr content ( Mo: Mo content (wt%)